It is a well-known fact that lithium ion secondary batteries, which have high energy density and whose discharge capacity does not significantly decrease, have been used for a power source for mobile tools such as mobile phones and laptop computers. In recent years, with the miniaturization of mobile tools, there also is a demand for the miniaturization of lithium ion secondary batteries to be mounted therein. In addition, with the development of hybrid cars, solar power generation, and other technologies as a measure to prevent global warming, etc., the application of supercapacitors having high energy density, such as electrical double-layer capacitors, redox capacitors, and lithium ion capacitors, has been increasingly expanding, and there is a demand for a further increase in their energy density.
An electrical storage device, such as the lithium ion secondary battery or the supercapacitor, has a structure in which, for example, a positive electrode, a negative electrode, and a separator made of a polyolefin or the like between them are arranged in an organic electrolytic solution containing a fluorine-containing compound, such as LiPF6 or NR4.BF4 (R is an alkyl group), as an electrolyte. Generally, the positive electrode includes a positive electrode active material, such as LiCoO2 (lithium cobalt oxide) or active carbon, and a positive electrode current collector, while the negative electrode includes a negative electrode active material, such as graphite or active carbon, and a negative electrode current collector, and, with respect to the shape, the electrodes are each obtained by applying the active material to the surface of the current collector and forming the same into a sheet. The electrodes are each subjected to high voltage and also immersed in the highly corrosive organic electrolytic solution that contains a fluorine-containing compound. Accordingly, materials for the positive electrode current collector, in particular, are required to have excellent electrical conductivity together with excellent corrosion resistance. Under such circumstances, currently, aluminum, which is a good electrical conductor and forms a passive film on the surface to offer excellent corrosion resistance, is almost 100% used as the material for a positive electrode current collector. Incidentally, as materials for the negative electrode current collector, copper, nickel, or the like can be mentioned.
One method for achieving the miniaturization and higher energy density of an electrical storage device is to thin a current collector that constitutes a sheet-shaped electrode. Currently, an aluminum, foil having a thickness of about 15 to 20 μm produced by rolling is commonly used as a positive electrode current collector. Therefore, the object can be achieved by further reducing the thickness of such an aluminum foil. However, in rolling, it is difficult to further reduce foil thickness on an industrial production scale.
Then, a possible aluminum foil production method to replace rolling is a method for producing an aluminum, foil by electrolysis. The production of a metal foil by electrolysis is performed, for example, by forming a metal film on a surface of a substrate such as a stainless steel plate by electroplating, followed by the removal of the film from the substrate. Such production is well known as a method for producing a copper foil, for example. However, aluminum is an electrochemically base metal, and thus electroplating is extremely difficult. Therefore, it is not easy to produce an aluminum foil by electrolysis. Patent Document 1 discloses, as a method for producing an aluminum foil by electrolysis, a method that uses an electrolytic bath containing 50 to 75 mol % of aluminum chloride and 25 to 50 mol % of an alkylpyridinium chloride or an electrolytic bath prepared by adding an organic solvent to such a bath. However, in this method, the chlorine concentration in a plating solution is extremely high. This leads to a problem in that during a plating treatment, chlorine contained in the plating solution reacts with moisture in the air to generate hydrogen chloride gas, causing the corrosion of equipment. Therefore, it is necessary to take a measure to prevent the generation of hydrogen chloride gas or a measure to protect equipment from corrosion due to the generated hydrogen chloride gas. Further, the method described in Patent Document 1 also has a problem in that the applicable current, density is about 2 A/dm2 maximum, and thus the film formation rate is low (when the applied current density is increased any further, the plating solution decomposes, etc., making it impossible to stably perform a plating treatment). The addition of an organic solvent, such as benzene or toluene, to the plating solution is expected to improve the film formation rate. However, these organic solvents have high toxicity and are dangerous because of high inflammability, and, therefore, it must be said that the addition of such organic solvents to a plating solution is problematic in terms of the ease of liquid waste disposal and safety.
In light of the above points, the research group of the present inventors has been vigorously conducting research on a method for producing a high-ductility, high-purity aluminum foil at a high film formation rate by electrolysis using a plating solution having a low chlorine concentration. As a result, in Patent Document 2, they have proposed a method in which an aluminum film is formed on a surface of a substrate by electrolysis using a plating solution containing at least (1) a dialkyl sulfone, (2) an aluminum halide, and (3) at least one nitrogen-containing compound selected from the group consisting of an ammonium halide, a hydrogen halide salt of a primary amine, a hydrogen halide salt of a secondary amine, a hydrogen halide salt of a tertiary amine, and a quaternary ammonium salt represented by the general formula; R1R2R3R4N.X (R1 to R4 independently represent an alkyl group and are the same as or different from one another, and X represents a counteranion for the quaternary ammonium cation), and then the film is removed from the substrate. According to Patent Document 2, the blending proportions of the dialkyl sulfone, the aluminum halide, and the nitrogen-containing compound in the plating solution for aluminum electroplating are such that per 10 mol of the dialkyl sulfone, the aluminum halide is preferably 1.5 to 4.0 mol, and the nitrogen-containing compound is preferably 0.01 to 2.0 mol. The plating solution of such blending proportions is useful for the production of a high-ductility, high-purity aluminum foil at a high film formation rate, etc. However, there remains room for improvement in terms of ease of handling, etc. Specifically, the plating solution of such blending proportions has the following issues. Because a dialkyl sulfone, which is used as a solvent component, has a high melting point (e.g., the melting point of dimethyl sulfone is about 110° C.), when an electroplating treatment is performed, unless the temperature of the plating solution is equal to or higher than the melting point of the dialkyl sulfone, the plating solution solidifies, making it impossible to perform the treatment. Further, because the plating solution is solid at a normal temperature of 25° C., the plating solution cannot be easily and smoothly prepared, disposed, etc. Therefore, there has been a demand for a method for lowering the melting point of the plating solution without significantly changing the blending proportions thereof.
In Patent Document 3 proposed by the research group of the present inventors, which relates to a method for lowering a melting point of a plating solution for aluminum electroplating, it is stated that in the case of a plating solution for aluminum electroplating containing a dialkyl sulfone and an aluminum halide, when the blending proportions of the two are such that the aluminum halide is 4 mol per 10 mol of the dialkyl sulfone, the melting point is lowered, allowing for an electroplating treatment even at 60° C. However, the plating solution described in Patent Document 3 does not contain the nitrogen-containing compound described in Patent Document 2. Regarding whether the melting point can always be lowered also in a plating solution containing the nitrogen-containing compound described in Patent Document 2 when the blending proportions of the dialkyl sulfone and the aluminum halide are such that the latter is 4 mol per 10 mol of the former, the study by the present inventors has revealed that it is not always possible.